The invention relates to an automotive belt tensioning device and configuration.
It is known for internal combustion engine belt drives to incorporate along the path of the belt, which acts as the drive element, at least one tension device, which helps to avoid undesirable belt slippage and compensates for belt stretching. The belt drivingly connects the crankshaft with the generator (as well as other possible secondary engine components which are driven by the crankshaft) and may be redirected by (i.e., trained around) various redirecting pulleys, e.g., a tension pulley on a tension arm. Spring preloading around the tension arm swivel bearing acts on this tension pulley. Because all secondary components customarily are driven by the crankshaft, it is preferable and customary to position the tension pulley on the belt""s nondriving side relative to the crankshaft.
A new development, used especially in automobile construction, incorporates the starter motor into the generator and starts the internal combustion engine by turning the crankshaft via the belt drive (as opposed to the usual method, where the internal combustion engine is started by the starter motor turning the flywheel via a starter motor pinion). When starting the engine using a combined starter/generator, the kinematic behavior of the belt drive is different than normal because the starter motor dives the crankshaft as opposed to vice-versa. Therefore, because the tension pulley is positioned on the ordinarily non-driving side of the crankshaft and the starter motor creates a relatively large and sudden torque, undesirable belt slippage and oscillations may result which the first tension pulley may not be able to suppress by itself, even with strong spring preloading.
The present invention solves these drawbacks or limitations of the prior art. The invention features a tension device having a pair of tension pulleys which are disposed within the engine drive system so as to tension the drive element. The tension device is positioned with the two tension pulleys engaging the drive member on opposite sides of the starter/generator pulley. Thus, one of the tension device pulleys engages the drive member at a position within the drive path corresponding to the xe2x80x9cslackxe2x80x9d position where a tensioner customarily might be located to tension the drive member when the crankshaft is driving the drive system, while the other pulley of the tension device is positioned at a location to take up slack in the drive member generated when the starter/generator is driving the drive member. Thus, the tension device of the invention provides proper tensioning under the differing kinematic conditions which arise when the starter/generator is driving the drive member (as opposed to when the crankshaft is driving the drive member).
Additionally, the pulley arrangement provides the drive element with enhanced stabilization and tensioning. The pulleys are positioned such that the applied forces result in oppositely directed moments relative to the tension device pivot axis, and the oppositely directed moments vary relative to one another depending on whether the drive element applies more force to the first tension pulley or the second tension pulley. Since both moments are transmitted through the swivel bearing, the two tension pulleys provide support for one another, thereby producing stable movement of the drive element even when the drive undergoes varying kinematic conditions. This is especially advantageous for a drive incorporating a combination generator/starter because the generator/starter drives the crankshaft during starting of the internal combustion engine, whereas the crankshaft drives the generator during normal engine operation. The two tension pulleys work together because a yielding movement by one of the tension pulleys causes the other tension pulley to move so as to produce an increase in tension.
In a drive system having a starter motor that is incorporated into the generator and that can be activated at will, it is advantageous to position the second tension pulley so as to engage the drive element on the driving side of the starting motor as a starting tension pulley. Such an arrangement can prevent or reduce slippage or undesirable strong oscillation of the drive element resulting from the strong force applied to the crankshaft by the starting motor via the drive element. The tension device arrangement of the invention is particularly suited to a belt drive which has a drive element that consists of at least one V-belt, grooved belt, or cog belt.
Various specific geometric configurations can be used to obtain the benefit of a strong force applied to one tension pulley pushing the other tension pulley into the drive element with increasing force. For example, the axes of the tension pulleys may be on the same side or on opposite sides of the swivel bearing; both axes may be on a single straight line passing through the swivel bearing; or each axis may be on a separate line passing through the swivel bearing, with the two lines forming an angle with each other. The particular geometry that is chosen should be matched to the path of the drive element in the drive, thereby ensuring that the tension device takes up relatively little space and can be positioned optimally within the drive.
In this regard, it is important that the first tension pulley preferably has a high tensioning capacity so that drive element length changes can be compensated for effectively and so that the increase in length that results when the second tension pulley yields can be taken up readily. Thus, if the first tension pulley compensates for a 5.5 mm increase in drive element length, for example, the second tension pulley should only introduce a significantly smaller increase in length, e.g., only 1 mm. The tensioning capacity of the first tension pulley should preferably be no less than 1:4. This can be achieved easily by appropriately designing the tension pulley lever arms so that they have a ratio of at least 1:4, preferably approximately 1:5.5. This guarantees that the first tension pulley has the high tensioning capacity mentioned above.
Preferably, the effective diameters of both tension pulleys are at least approximately the same, and preferably the drive element passes through the tension device in such a way it makes approximately equal contact with both tension pulleys.
The tension device preferably is spring preloaded as is customary. The spring preloading applies force to the first tension pulley, viz., the one with the high tensioning capacity, thereby forcing it in the direction of increasing tension. In order to damp drive element oscillations effectively and also in order to avoid slippage, it is preferable to provide rotational damping in the swivel bearing or for the tension lever.
In order to avoid damage to the drive in the case of extreme conditions that may occur, the tension arm should have a limited total swivel angle within which it should be able to carry out its working cycle. A total swivel angle of approximately 30xc2x0 is preferable. If the total swivel angle in the swivel direction which causes a reduction of tension is limited by a mounting end stop, the drive element can be easily installed and removed.